When we do departures, our procedure is to show that we have removed the steering pin. {As most, no doubt are} But I have to say, sometimes getting the flight crews attention is sometimes hard as they are completing their pre taxi checks and are often looking down. So I would imagine, that some dont even look to see where the tug or tow bar are. If we sign off on the headset and say our courtesy good-byes, I would say thats good enough for the drivers.
I think this sounds like, as all us aviation profesionals know, all the holes in the swiss cheese lined up!! Just an unlucky mistake.